DOE OSTI2015
The direct conversion of sunlight into electricity represents one of the most important general means for sustainable energy production. Most modern photovoltaic cells are based on some form of semiconductor material such as silicon that is described by a characteristic energy or ''bandgap''. For solar rays with photon energy lower than the bandgap no absorption occurs and thus no energy is generated. For solar rays with photon energy greater than the bandgap, light may be absorbed to create a pair of electrical charges but only one bandgap of energy is created, leaving any additional energy lost as heat. Thus low bandgap materials can use a great part of the spectrum but can lose much of that energy; high bandgap materials fail to capture much of the spectrum. Thus there is a limit to the efficiency of such a solar cell that turns out to be about 32%. This limit is known as the Shockely-Queisser Limit. The Columbia EFRC program is dedicated to exploration of concepts that in principle can provide for efficiencies beyond this limit. One concept that this EFRC has explored for enhancing the efficiency of solar photovoltaic cells is called “Singlet Fission.” In this concept the absorption of light rays with photon energy at least twice the value of the basic bandgap for the system can produce two pairs of electrical charge carriers. If properly implemented this in principle can significantly reduce the energy lost as heat and give rise to solar cell efficiencies greater than the Shockley-Queisser limit. The problem is that there are virtually no materials that can undergo this process effectively. We have developed new materials that have demonstrated singlet fission efficiencies of almost 100%. We understand how these materials work and we have learned how to design many more systems in the future. So far we have only demonstrated the basic capability for efficient singlet fission. Much more work will be required to design and engineer specific materials that can be used practically in a solar cell system. In addition much work will be required to envision and demonstrate effective device structures that can utilize this concept. However these discoveries do provide the basis for an entirely new set of opportunities for more efficient solar energy generation moving beyond the Shockley-Queisser limit. A second part of the EFRC research program has been to investigate the material and device properties of an entirely new set of materials based on two-dimensional sheets (“ultra-thin”) with thicknesses of only one atom, or a single molecule or just a few atoms. These materials can exhibit conducting, insulating, and semiconducting character and thus they can form the basis for entirely new types of electrical devices. Recent fundamental investigations of these materials, at Columbia and elsewhere, demonstrate clearly that the flow of electrical charges in these systems is fundamentally different from the nature of electrical current flow in conventional materials. This fact presents many possibilities for new photovoltaic device concepts. The EFRC research team has achieved world leadership in the creation and understanding of these materials and in developing the fabrication techniques necessary to create useful devices from them. We have developed the basic fabrication methodology to build structures of these materials into complex device structures, layer by layer. Our EFRC research team has pioneered the synthesis and understanding for graphene, perhaps the simplest of these materials. Graphene can function as a highly transparent conducting material, capable of funneling an electrical charge over reasonable distances without significant energy loss. The EFRC program has also pioneered the development of ultra-thin sheets that function in a way analogous to semiconductor materials as well as sheets that act as electrical insulators. These developments therefore enable the construction of solar cells based on totally different physics from conventional cells. Because the electrons in these ultra-thin sheets interact strongly they will exhibit behaviors quite different from conventional materials with potential to operate at efficiencies beyond the Shockley-Queisser limit. In our EFRC program we have laid out many of the fundamental properties of these materials including the development of unique fabrication techniques. We discovered several new effects that demonstrate strong electron coupling. We have demonstrated the first solar cell that can actually generate electrical power at a high basic efficiency from these fundamentally new materials. There is much more science and technology required before these devices can become practical, but there is also very strong activity worldwide to build electronic devices from these materials, thus providing infrastructure and technical capability to develop these concepts.